The problem is its a short corridor about 10 foot long from top of staircase to base of other staircase with a door in the midle to another office, and the staircase is open on the upper treads, and building regs dont allow a door to be on the top step of a staircase or within a certain distance of a bottom step, eg there must be a landing area.

So fire safety is not my area of expertise however I can give you my opinion. If the staircases are two seperate means of escape we may require a fire door ( not to be confused with a fire exit) to be fitted to provide seperation so if one staircase is compromised by smoke it will prevent the other going the same way.
Difficult to comment to much without seeing it.

As for powers well we can prohibit the use of a building or require you to comply with an improvement letter and anything in between. My advice would be talk to the FP officer and see whst his reasons are and where he may be willing to compromise.
Just for interest a fire exit must lead to a place of ultimate safety not just fresh air

can confirm the rro2005(fire certificates are long gone)
speak with the fire protection officer there are clear rules on escape routes and fire doors so he should be able to say if this is a must or a nice to (good practice)
yes companies are now responsible for there own fire risk assessments but building regs and legislation still cover the building works

Went to estimate a job last week, in a an old 3 storey multi occupancy building, that has been trading for numeous years, fire officer turns up and says he wants a fire door fitted into a short corridor between two staircases, the problem being there is no space for the door to be fitted.

The Regulatory Reform Order means it’s now the responsible person’s duty to make an assessment, but it doesn’t mean you can assess something as safe if it’s not, and there are some requirements – see s14, and enforcement powers, see s27.http://www.legislation.gov.uk/uksi/2005/1541/contents/made